There is a continuing interest in the industry for self-extinguishing and non-burning resinous materials, for example, for surface coatings, castings, moulded articles, and in particular for laminates for use in electrical equipment. The excellent physical, chemical and electrical properties of cured polyepoxides make these compounds extremely suitable for the above applications. However, the usual polyepoxides are, as most organic materials, flammable. Therefore, many attempts have been made to improve the flame retardancy of this class of resins. One of the most widely investigated fields of research to improve the flame retardancy has been the incorporation of chlorine or bromine containing resin compounds as halogenated polyepoxide compounds or halogenated curing compounds. Further, also the use of chlorine and bromine containing additives has been widely studied. Nowadays, however, from an environmental point of view the use of halogenated compounds is less desired. Thus, there is an active interest to look for non-halogenated fire retardants.
The use of red phosphorus in a process for imparting flame proofing properties to polyepoxides is known from U.S. Pat. No. 3,477,982. It has been described in this document that resin compositions having a good flame retardancy can be obtained by mixing especially glycidyl polyethers of polyhydric phenols and polycarboxylic anhydrides or amino compounds having at least two amino hydrogen atoms per molecule with finely divided red phosphorus. Preferably 6 to 25 parts by weight of finely divided red phosphorus per 100 parts by weight of polyepoxide is used (column 1, line 70). From Table IX it is indeed clear that, in order to obtain good extinction times at least 6 weight percent of phosphorus is necessary. Further, in the other examples usually 8 to 11 percent by weight on resin composition is used. The compositions described in this document, either as such or in combination with a solvent, may be used for the impregnation of glass fiber materials for the preparation of prepregs, which can be used for making laminates.
In European patent application 428,863, it has been described that in view of the high amounts of red phosphorus necessary to reach the required flame retardancy (more than 15 percent based on resin) it is hardly possible to reach the requirements with respect to the E-corrosion resistance (page 4, lines 32 ff.). It is further indicated that the combination of red phosphorus and metal oxides, carbonates or hydroxides not results in epoxy resin systems having the UL 94V required V-0 flame resistancy. In Example 6 epoxy resins are prepared from an epoxidized novolac in combination with a novolac curing agent. The results of the flame retardancy properties of the resins made are shown in Table 6. It appears that acceptable results are only obtained when using large amounts of red phosphorus (15 percent, Example 12) or by using the combination of red phosphorus and alumina (7.5 percent each, Examples 13 and 14). It is desirable to solve the problem of the high amounts of red phosphorus to obtain the required flame resistancy.